To improve a low frequency response of a loudspeaker, and implement a rich and smooth sound effect, a resonance frequency f0 of the loudspeaker needs to be reduced, and a frequency band width of a component needs to be increased. In the prior art, there are usually two methods. A first method is to expand a back cavity of the loudspeaker, for example, increase an external dimension of a sound box, so that the loudspeaker has a larger back cavity. A second method is to add a cavity expansion material. The cavity expansion material is added to implement virtual expansion of the back cavity of the loudspeaker. The first method is usually used in a loudspeaker with a large volume or a loudspeaker whose volume can be expanded. However, for a loudspeaker whose back cavity is difficult to be physically expanded, in particular, for a micro loudspeaker, virtual cavity expansion is usually implemented by adding the cavity expansion material.
The cavity expansion material is usually a porous, loose, and breathable object, such as natural zeolite, activated carbon, and various types of foaming bodies. Because the cavity expansion material has a large quantity of through micropores that are connected to each other, the micropores can breathe to some extent. When a sound wave is incident to a surface of the porous material, air vibration in the micropore is caused. Due to a friction resistance, a viscous resistance of air, and a heat conductivity function, a considerable amount of sound energy can be converted into thermal energy, to absorb sound and expand the cavity.
However, there are many problems in the cavity expansion material in the prior art. For example, zeolite materials are made into small balls of a micron size. However, such type of material has a low strength, and is fragile during actual use. In addition, the small balls need to be packaged into a dedicated enclosure/cavity, and the enclosure is packaged by using a dedicated cover. Raw material has a complex manufacturing technology, a low strength, high assembly difficulty, high costs, and selectivity of the enclosure. Consequently, universal applicability is low, and application of the material is limited.